POK transcription factors have been recognized as critical developmental regulators and have been directly implicated in human cancer. As an example BCL6 and PLZF are critical players in the pathogenesis of Non- Hodgkin's lymphoma and Acute Promyelocytic Leukemia, respectively. We have characterized a new important member of this family, POKEMON/LRF, that shares structural similarities with BCL6 and PLZF and plays a critical and essential role in embryonic development, hematopoiesis and hematological malignancies. Our current analysis demonstrates that this protein plays a key and pleiotropic role in controlling cellular differentiation and pathogenesis of solid tumors, acting as a proto-oncogene essential for neoplastic transformation, but also, intriguingly, as a putative tumor suppressor depending on the cell context and the genetic milieu. We propose to define POKEMON's role in tumorigenesis and to determine the regulatory mechanisms that control its function with the following Specific Aims: To define the role of POKEMON/LRF as a proto-oncogene. Our analysis of the expression of LRF in human cancers shows an ectopic expression of LRF in hepatocellular carcinoma, while it has recently been shown that the LRF gene is amplified in non small cell lung cancer (NSCLCs). We propose to determine the mechanisms and role of LRF in solid tumorigenesis including a direct genetic approach in the mouse. To determine the role of POKEMON/LRF as a putative tumor suppressor gene. Our recent finding that Lrf can oppose Notch signaling suggested that LRF could act as a tumor suppressor in particular cellular contexts. Our analysis in Pokemon/Lrf conditional KO mutants indicates that this is surprisingly the case. Here we propose to perform a thorough analysis of the tumor suppressive role of LRF both in vivo and in vitro. We will study the mechanisms by which loss of Lrf triggers tumorigenesis and the cooperative mechanisms of loss of Lrf and the Pten tumor suppressor in prostate cancer progression. To determine the oncogenic capability of POKEMON/LRF-targeting miRNAs. MicroRNAs (miRNAs) have recently come into focus as a novel type of post-transcriptional regulatory element. We recently reported that LRF is under the miRNA post-transcriptional regulation. The current proposal aims at studying the functional cross-talk between miR-20a family members and LRF both in vitro and in vivo in prostate-specific miRNA transgenic mouse models. To define the role of POKEMON/LRF post-translational regulatory mechanisms. Post-translational modifications, such as phosphorylation and acetylation, play a pivotal role in modulating the functions of transcription factors. We have identified specific phosphorylation and acetylation sites in LRF and we have demonstrated that GSK32 and SIRT1 are responsible for these modifications. We propose to study the consequence of such biochemical modifications on POKEMON/LRF function both in vitro and in human tumor samples.